1. Field of the Invention
The present invention relates to the field of electrography and, more particularly, to improvements in magnetic brush apparatus and systems for developing electrostatic images.
2. Description of the Prior Art
In commonly assigned U.S. Pat. No. 4,473,029, there is disclosed an electrographic development system comprising a magnetic brush applicator and a two-component developer. The magnetic brush applicator comprises a cylindrical sleeve having concentrically positioned therein a cylindrically-shaped multi-pole magnetic core piece. Means are provided for rotating the magnetic core piece at a relatively high speed (e.g. 1000-3000 rpm) and, optionally, for rotating the sleeve at a slower speed (e.g. 50-100 rpm). The developer comprises a mixture of thermoplastic toner particles and "hard" magnetic carrier particles of high coercivity (&gt;500 gauss) and induced magnetic moment (&gt;5 EMU/gm in a field of 1000 gauss). The toner particles adhere to the carrier particles by triboelectric forces. During rotation of the magnetic core piece, the developer is transported along the sleeve's outer surface from reservoir to a development zone. There, the developer comes into contact with a latent electrostatic image, and toner is stripped from the carrier particles to effect image development. Following image development, the partially denuded carrier particles are stripped from the sleeve and returned to the development reservoir for toner replenishment.
In electrographic development systems of the type described in the above-noted patent, each of the developer's carrier particles is itself a tiny permanent magnet and behaves as such on the brush sleeve surface. Thus, as the magnetic core piece rotates at high speed within the sleeve, each carrier particle on the outer surface of the sleeve continuously flip-flops, end-for-end, in attempting to align itself with the rapidly changing magnetic field. While this fast-changing magnetic field and the resulting flipping action of the carrier results in excellent transport and mixing of the developer on the brush sleeve surface, it has the drawback of undesirably increasing the temperature of the developer mass. When the developer contains toner having a relatively low glass transition temperature, as is necessary in high speed copiers to effect rapid fusion of the toner to the copy sheet, toner agglomerations can form which have an adverse effect on toner replenishment and ultimately on image quality.
The developer heating problem noted above worsens as the time during which the developer is subjected to the high field gradient produced by the rotating core piece increases. Were it possible to feed the developer to the brush sleeve at a point just upstream of the development zone and to remove the developer from the brush immediately after development, the developer heating problems would be alleviated to a great extent. Unfortunately, due to many considerations, it is often impractical to supply developer to the brush sleeve at a location less than about 60.degree. upstream of the development zone and to remove the developer any sooner than about 60.degree. downstream of the developer zone. Unfortunately, during this 120.degree. (or more) angular range that the developer is subjected to the high field gradient required to effect good development at the development zone, the developer heating problem arises.
Other technical problems associated with development systems of the rotating core/hard magnetic carrier type are those of high torque requirements and developer skiving difficulties. The torque requirements are high due to the need to rotate the core piece at high speed within the magnetic developer mass which, as indicated above, is in relatively close proximity to the core piece over an angular range of at least 120.degree. of the core rotation. The magnetic field between the core piece and developer acts as a drag on the core piece, increasing the work required to rotate the core and, hence the power requirements of the overall copying apparatus. Skiving of the developer (e g. to effect toner replenishment) can be difficult because the developer must be physically stripped from the brush sleeve while in the presence of the same high magnetic field gradient required to effect good image development.
Since the severity of all of the above-noted problems is closely related to the magnetic field strength produced by the rotating core piece, an obvious solution might be to reduce the field strength of the core magnets. However, as copy throughput increases, one finds that this approach gives rise to another technical problem, namely, "carrier pickup," i.e., an undesired deposition of carrier particles in the developed image. Carrier pickup can produce devastating effects in the form of scoring of the recording element and other copier components (e.g. fusing and/or transfer rollers). To minimize carrier pickup, it is desirable to maintain a magnetic field at the development zone of at least 1000 gauss and, at this field strength, the aforementioned disadvantages return.